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bInorganic polyphosphate (polyP), a linear polymer of hundreds of phosphate residues linked by ATP-like phosphoanhydride bonds, is found in all organisms and performs a wide variety of functions. This study shows that polyP accumulation occurs in Mycobacterium tuberculosis upon exposure to various stress conditions. M. tuberculosis possesses a single homolog of ppk-1, and we have disrupted ppk-1 in the M. tuberculosis genome by allelic replacement. The mutant strain exhibited negligible levels of intracellular polyP, decreased expression of sigF and phoP, and reduced growth in the stationary phase and displayed a survival defect in response to nitrosative stress and in THP-1 macrophages compared to the wild-type strain. We report that reduction in polyP levels is associated with increased susceptibility of M. tuberculosis to certain TB drugs and impairs its ability to cause disease in guinea pigs. These results suggest that polyP contributes to persistence of M. tuberculosis in vitro and plays an important role in the physiology of bacteria residing within guinea pigs.

Toxin-antitoxin (TA) systems are highly conserved in members of the Mycobacterium tuberculosis (Mtb) complex and have been proposed to play an important role in physiology and virulence. Nine of these TA systems belong to the mazEF family, encoding the intracellular MazF toxin and its antitoxin, MazE. By overexpressing each of the nine putative MazF homologues in Mycobacterium bovis BCG, here we show that Rv1102c (MazF3), Rv1991c (MazF6) and Rv2801c (MazF9) induce bacteriostasis. The construction of various single-, double-and triple-mutant Mtb strains reveals that these MazF ribonucleases contribute synergistically to the ability of Mtb to adapt to conditions such as oxidative stress, nutrient depletion and drug exposure. Moreover, guinea pigs infected with the triple-mutant strain exhibits significantly reduced bacterial loads and pathological damage in infected tissues in comparison with parental strain-infected guinea pigs. The present study highlights the importance of MazF ribonucleases in Mtb stress adaptation, drug tolerance and virulence.

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Aspergillus spp. are increasingly recognized as major fungal pathogens in immunocompromised or neutropenic patients, and Aspergillus fumigatus is responsible for nearly 90% of cases of invasive pulmonary aspergillosis (IPA) (7). As the incidence of AIDS, aplastic anemia, and organ transplantation increases and the use of chronic glucocorticoid treatment and aggressive antineoplastic chemotherapy regimes becomes more frequent, the number of patients susceptible to Aspergillus infection is rising. In immunocompromised or neutropenic patients, IPA, the most common form of the disease, is characterized by hyphal invasion and destruction of pulmonary tissue. Dissemination of Aspergillus infection to other organs occurs in approximately 20% of IPA cases (4). In spite of correct diagnosis and treatment, IPA results in patient mortality of greater than 80%. The mortality rate among bone marrow transplantation patients can be as high as 95% (26). Early empirical treatment with antifungal drugs, such as amphotericin B (AmB), reduces the mortality rate. However, AmB is considered highly nephrotoxic (18) and has often been found inadequate for complete elimination of the infection in the immunosuppressed subjects. Because invasive aspergillosis is extremely rare in immunocompetent individuals, therapy aimed at strengthening the host's immune response to the organisms offers a promising new approach in the treatment of this disease.Host defense against Aspergillus infections is considered to be mediated by macrophages, neutrophils, and polymorphonuclear cells (PMNs) (6,16,23,24,25,28). The respiratory tract appears to be the portal of entry in most cases of IPA (3). Therefore, there has been an extensive search for molecules in the lung which can selectively enhance the contribution of the innate immune mechanism of phagocytes against Aspergillus infection. Lung surfactant proteins SP-A and SP-D have potent chemotactic activity for various subsets of mononuclear leukocytes and have been shown to enhance phagocytosis and production of superoxide anion by macrophages and neutrophils (29). SP-A and SP-D, which belong to a family of proteins called collectins, are also known to interact with carbohydrate structures present on the surfaces of a wide range of pathogens, such as viruses, bacteria, and fungi, via their carbohydrate recognition domains (CRDs) and to enhance phagocytosis and killing by neutrophils and macrophages (22,29). Collectins are composed of subunits, each of which contains a collagen-like triple-helical region, followed by an ␣-helical, trimerizing neck region and three CRDs at its C-terminal end. Six of these trimeric subunits make up the overall structure of SP-A, while SP-D is composed of a cruciform structure with four arms of equal lengths (10). Mice deficient in SP-A were observed to be less effective in clearing Staphylococcus aureus and Pseudomonas aeruginosa and were more susceptible to lung inflammation and splenic dissemination of group B streptococci (13-15).We have previously shown that SP-A and S...